In a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS), a device for ionizing a sample and an ICR cell for detecting ions are relatively distant from each other to prevent the magnetic field applied to the ICR cell from affecting the ionization device. Because of this structure, the ions generated by the ionization device spatially diffuse due to the mass difference of the ions as they travel to the ICR cell, although they are initially propagated with the same energy.
In general, the ICR cell traps the propagated ions by a method called gated trapping. In the gated trapping method, the ICR cell is configured such that incoming ions can travel freely by lowering the electric potential of an electrode at the side where the ions come in and by raising the electric potential of an electrode at the opposite side so that they cannot pass. When the ions to be detected enter the ICR cell, the electric potential of the incoming side electrode is increased to confine the ions in the ICR cell. However, since the ions reaching the ICR cell are spatially diffused due to their mass difference, only some of the ions can be trapped in the ICR cell and measured with this method. That is to say, it is difficult to detect a broad mass range at once.